Add support for filling layout

[cpsievert]

TODO

• Make sure this works with Quarto
• Do a test run through more widgets
• Make sure specified height/width works sensibly
• Finish updating examples to leverage fill
• Remove the examples/fill/app.py?
• Clean-up/simplify resize logic
• Make sure we aren't mutating objects
• Update example requirements
• Make it possible to opt out of fill in the output_widget

[jcheng5]

My only concern for the record--should not hold up merging--is that this is assuming that the vast majority of widgets are going to want to fill. This might turn out to be true, but if not, then we'll have to be ready to respond.

[jcheng5]

LGTM other than async/await